Let's do some math ..  Rounding examples of 7 random hypothetical purchases:
  Case Tag  
Price  
Sales  
Tax %  
Raw Total  
Incl. Sales Tax  
Rounded  
to Penny  
Rounded  
to Nickel  
Rounded  
to Dime  
  Purchase #1 $229.66   8.742%   $249.7369   $249.74   $249.75   $249.70  
  Purchase #2 $180.49   4.470%   $188.5579   $188.56   $188.55   $188.60  
  Purchase #3 $121.26   9.471%   $132.7445   $132.74   $132.75   $132.70  
  Purchase #4 $135.59   6.798%   $144.8074   $144.81   $144.80   $144.80  
  Purchase #5 $130.88   3.842%   $135.9084   $135.91   $135.90   $135.90  
  Purchase #6 $81.80   9.551%   $89.6127   $89.61   $89.60   $89.60  
  Purchase #7 $60.31   12.684%   $67.9597   $67.96   $67.95   $68.00  
  7 purchase totals = $939.99       $1,009.3275   $1,009.33   $1,009.30   $1,009.30